The major goal of this research program is to identify the gene products of Friend and Rauscher spleen focus forming viruses (SFFV), and to obtain evidence concerning their leukemogenic roles. One approach has been to clone the Rauscher SFFV into fibroblasts and to compare viral gene products in the resulting cell lines with the gene products in the resulting cell lines with the gene products of Friend SFFV. Our work already accomplished suggests that the SFFVs are closely related to each other and to the dual tropic MuLVs which are env gene recombinants of ecotropic and xenotropic viruses. Although F-SFFV and R-SFFV have nearly identical recombinant env genes, they differ dramatically in their gag and pol genes. The cell surface localization of SFFV encoded antigens is also being analyzed. Another approach involves purification and structural analysis of the recombinant-type gp55 glycoprotein encoded by F-SFFV. A third approach involves isolation of F-SFFV mutants and analysis of these mutants for leukemogenicity.